Flip chip microelectronic assembly is already established in the semiconductor industry. The assembly process involves the direct electrical connection of face-down electronic components onto substrates, circuit boards or carriers by means of conductive bumps on the chip bond pads. In contrast, older technology involves wire bonding of each bond pad of face-up chips to substrates, circuit boards or carriers. The advantages of flip chip packaging over older technology include smaller size, better performance, increased flexibility, reliability and lower cost. For example, eliminating packages and bond wires reduces the required board area by up to 95%, and requires far less height. Weight can be less than 5% of packaged device weight. In fact, the flip chip can be even smaller than Chip Scale Packages (CSP) because its size is the size of a chip. Flip chip materials are also becoming more widely available, further lowering production costs.
A conductive bump serves several functions in the flip chip assembly. Electrically, the bump provides a conductive path from chip to substrate. The bump also provides a thermally conductive path to carry heat from the chip to the substrate. In addition, the bump provides part of the mechanical mounting of the chip to the substrate. Finally, the bump provides a spacer, preventing electrical contact between the chip and substrate conductors, and acting as a short lead to relieve mechanical strain between board and substrate.
There are several ways of forming bumps on semiconductor wafers. These include solder bumping, plating and stud bumping. The present invention relates to stud bumping, which bumps wafer dice by a wire bonding technique that is modified from the older wire bonding technology. This technique makes a ball for wire bonding by melting the end of a wire to form a sphere. The ball is attached to the chip bond pad as the first part of a wire bond. To form bumps instead of wire bonds, wire bonders are modified to break off the wire after attaching the ball to the chip bond pad. The ball, or “stud bump” remaining on the bond pad provides a permanent connection to the underlying metal on the chip.
Traditionally, gold wire is used in stud bumping. However, there has recently been increased interest in using copper wire instead. Copper bumps have been found to offer increased reliability, extended temperature range, greater mechanical strength, higher connection density, improved manufacturability, and better electrical and heat-dissipating performance. Nevertheless, a challenge faced in copper stud bumping is that copper reacts with oxygen and oxidizes at the high temperatures that stud bumping processes are carried out. The formation of copper oxide, which is non-conductive, result in defects in the final product, and this needs to be avoided as far as possible. It would thus be desirable to have access to an apparatus that allows copper stud bumping to be performed at a high temperature while reducing the risk of oxidation of the copper bumps.